In installations for producing nitrogen under pressure, the nitrogen is usually produced directly at the pressure of use, for example between 5 and 10 bars. Purified air, compressed slightly above this pressure, is distilled so as to produce the nitrogen at the top of the column and the reflux is achieved by expansion of the “rich liquid” (liquid at the base of the column formed by air enriched with oxygen) and cooling of the condenser at the top of the column by means of this expanded liquid. The rich liquid is thus vaporized at a pressure of between about 3 and 6 bars.
If the size of the installation justifies this, the vaporized rich liquid is passed through an expansion turbine so as to maintain the installation in the cold state but, often, this refrigerating production is excessive, which corresponds to a loss of energy. In the opposite hypothesis, the cold state is maintained by an addition of liquid nitrogen coming from an exterior source, and the vaporized rich liquid is simply expanded in a valve and then travels through the thermal exchange line serving to cool the initial air. Consequently, here again, a part of the energy of the vaporized rich liquid is lost.